A liquid crystal display device represents a transmission type display device having a light source. The transmission type liquid crystal display device requires a separate light source since a liquid crystal panel itself does not emit light. A direct-viewing type liquid crystal display device has a light source called “back light” mounted behind the liquid crystal panel, which light source is usually a fluorescent lamp generally called a cold cathode tube. A projection type liquid crystal display device called generally a liquid crystal projector which uses a lamp such as a halogen lamp and metal halide lamp to obtain brightness of an image projected on a screen.
FIG. 15 shows a general construction of a direct-viewing type liquid crystal display device. In FIG. 15, there is shown a liquid crystal display portion 201, a backlight 202, a backlight control portion 203, a display control portion 204 and an input 205. A picture signal to be displayed on a liquid crystal display portion 201 is input in the form of a signal YPbPr (a luminance signal and a color difference signal) from the input 205. The display control portion 204 performs control operations for displaying the input picture signal on the liquid crystal display portion 201. In practice, the display control portion 204 performs, for example, conversion of the signal YPbPr into a signal RGB, rearrangement of picture signals in accordance with the method of driving the liquid crystal display portion 201 and adaptive gamma correction for the liquid crystal display portion 201. The backlight 202 is a light source from which the liquid crystal display portion 201 obtains luminance. The backlight control portion 203 drives the backlight and controls the operation of the backlight so that it emits light always at a specified level of brightness. This backlight control portion 203 may have a dimming function, which has a plurality of preset voltage values (or current values) to obtain corresponding predetermined values of brightness of the backlight and regulates the brightness of the backlight 202 by selecting the voltage value according to a command signal given for example from a personal computer.
FIGS. 16A and 16B show the relationship between an average level of brightness of a picture signal to be displayed and a screen brightness of an image display device. Two kinds of the screen brightness are preset as brightness control values. Characteristic 16-1 relates to the brightness of the backlight, which is obtainable when a user-viewer selects ┌bright┘ mode. It shows that the display screen brightness 420 candelas is obtainable when a white signal is displayed on the screen. Characteristic 16-2 relates to the brightness of the backlight, which is obtainable when the user selects ┌normal┘ mode. It shows that the display screen brightness 260 candelas being is obtainable when a white signal is displayed thereon. As apparent from FIGS. 16A and 16B, the user-viewer can change, by using the dimming function, the current brightness of the display screen to the other brightness which value is always constant independent of a picture signal to be displayed thereon.
The display screen luminance (brightness) of the display device is determined as a product of multiplication of the transmittance of the liquid crystal display portion 201 by the brightness of light emitted from the backlight 202. As described above, since the brightness of the backlight 202 is constant independently of picture signals, the gradation of each picture signal to be displayed depends upon only the transmittance of the liquid crystal display portion 201. In other words, the display capability of the image display device can be determined by the dynamic range of the liquid crystal display portion 201 (i.e., the ability to display a white signal and a black signal respectively).
In recent years, there have been proposed a number of methods for improving the image quality and the visibility of liquid crystal display devices by dynamically regulating the contrast of picture signals and the brightness of the light source in accordance with picture signals varying with time.
Patent document 1 discloses an example of conventional improving method by dynamically regulating a backlight, which is described below with reference to FIG. 17. This method is featured by using an average brightness detecting circuit 206 and a backlight control portion 207 shown in FIG. 17. An average brightness detecting circuit 206 detects an average brightness level of a picture signal. With a high average brightness level detected by the circuit 206, the backlight control portion 207 controls the backlight 202 to decrease its luminance of the light. The relationship between the average brightness of the picture signal and the brightness of the display screen is shown in FIG. 18. As the brightness of the display screen is thus regulated in accordance with the average brightness level of a picture signal, it is possible to effectively prevent the display screen from having excessive brightness or darkness, thereby providing a display image that the user can view with ease and pleasure. This feature enables the user to visually sense an apparently widened dynamic range of the display in comparison with the case of merely maintaining a constant brightness of the backlight. In other words, the contrast of the image is increased on a dark screen and a bright screen respectively. Furthermore, if an image contains a bright part such as a metallic element on a dark background, it can have a high quality with increased luster provided by the effect of the increased brightness of the metallic part.
The patent document 2 discloses a conventional improvement of the display image quality of a display device by dynamically regulating both the contrast of a picture signal and the brightness of a backlight in a certain correlation. The method disclosed in the document 2 increases a dynamic range of a picture signal according to average brightness and shifts a level of the picture signal according to an offset value. This may cause the level shift of a visually sensible brightness of the image on the screen, so the backlight brightness is at the same time regulated by dimming control to absorb the possible level shift of visually sensible brightness. The visual contrast of an image can be improved by the above processing.
Patent document 3 discloses another example of a conventional method of improving the quality of an image by dynamical and correlative control of both the contrast of a picture signal (amplitude modulation of the signal) and the brightness of the backlight (output modulation of the light source). The conventional method of the document 3 is such that a dark level of a picture signal is detected and, if the duration of the detected dark level exceeds a threshold value, the output level of the light source is reduced and, at the same time, the dynamic range of the picture signal is enlarged. If the dark level duration does not exceed the threshold value, no modulation is done for both the output light and the picture signal. The above regulation can reduce the unevenness of the dark-level image area with no affection on the bright-level image area.
However, the method of the document 1 for improving the display image quality by increasing the brightness of the backlight when the average brightness level of a picture signal is low involves the following problem. Namely, the liquid crystal display portion 201 may have a leakage of light from the backlight even when displaying a black signal thereon. The light leakage causes a phenomenon called “loss of true black” which makes the display image generally whitish, considerably degrading the image displayed. Therefore, an increase in the intensity of the backlight is accompanied by losing the true black image. The brightness of the light source can not always improve the contrast of the display image.
The method of the document 2 for improving the display image quality by increasing a dynamic range of a picture signal also involves the following problem. Namely, if the dynamic range of a picture signal containing a picture of collapsed gradation is simply widened, the defective parts of the picture are emphasized and recognized as defects by the user. If the dynamic range of a picture signal with a noise component superposed thereon is increased, the noise component is also emphasized and recognized as a defect by the user.
The method of the document 3 for improving the display image quality by detecting the duration of the black level of the picture signal and simultaneously modulating both the picture signal and the backlight output also involves the following problem. Namely, the modulation of both the picture signal and the backlight output is conducted only based on information of the black level of the picture signal and without referring to other kinds of information such as (for example average brightness of) the picture signal, so the modulation is not made for the picture signal if it has no black level, i.e., the display image cannot be improved. Like the method of the document 2, the amplitude modulation of the picture signal emphasizes the defectively gradated parts and the noise component superposed on the picture signal.
(Patent Document 1)
Japanese Laid-Open Patent Publication No. 8-201812
(Title: Liquid Crystal Display Device)
(Patent Document 2)
Japanese Laid-Open Patent Publication No. 2001-27890
(Title: Image Display Device and Image Display Method)
(Patent Document 3)
Japanese Laid-Open Patent Publication No. 6-102484
(Title: Image Display Device Using a Spatial Light Modulating Element and Image Display Method Using the Same)